A reported cannabis-positive test involving an Air India pilot has brought attention to the complexities of detecting impairment in aviation. While a positive test indicates the presence of THC, it does not definitively establish when the substance was consumed or whether the individual was impaired at the time of duty.
The science surrounding cannabis impairment is complicated by factors such as the dose, method of consumption, frequency of use, and individual biology. Experts note that unlike alcohol, where blood alcohol concentration has a more direct correlation with impairment, the relationship between THC in the bloodstream and impairment is less clear. THC can be rapidly eliminated from the bloodstream after smoking, and its presence days or even weeks later, particularly in frequent users, does not necessarily indicate ongoing impairment.
Studies suggest that impairment from smoking cannabis typically lasts three to five hours. When consumed in edibles, the effects can last longer, generally six to eight hours or more. In such cases, the metabolite 11-hydroxy-THC may be more closely linked to cognitive impairment than THC itself.
This lack of a universally accepted, definitive test for current cannabis impairment presents a challenge for regulators. For instance, after legalizing recreational cannabis in 2018, Transport Canada implemented a rule prohibiting flight crews and air traffic controllers from consuming cannabis for at least 28 days before duty, aiming to address these uncertainties. Airlines are also permitted to enforce stricter policies.
In the United States, a 2026 National Transportation Safety Board (NTSB) study examining pilots involved in civil aviation accidents between 2018 and 2022 found that THC was the primary drug detected among pilots with positive tests. However, the NTSB emphasized that this study focused on the presence of drugs, not on whether the pilots were impaired at the time of the accidents.